Prospekt MAURER Seildaempfer Englisch

8/9/2019 Prospekt MAURER Seildaempfer Englisch

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MAURER Stay Cable Dampers


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Vibration of Stay Cables

Rain in combination with wind

can cause considerable vibrations

in stay cables. In unfavourable

circumstances, the magnitude of

the vibrations of the stay cables

can endanger the total bridgestructure, in particular when the

vibrations match the natural

frequency of the bridge structure.

But as a minimum, undesired

vibrations may lead to fatigue

problems, reducing the design

life of stay cables, and also

shortening the maintenance

interval.

Modes of vibrations of stay cables, and their natural frequencies

Principle sketch of a cable of length L that vibrates in the 1st mode. Distance of the cable

damper from the edge is xd, where a cable amplitude yd prevails. Maximum amplitude in

the centre is Y1. Tension force in the cable is F, mass of the cable per meter is .

What can be done about undesired vibrations?

Simply speaking, when required, a so called cable vibration damper shall be attached to the

stay cable, to reduce the amplitudes of the vibration to a tolerable level.

Instead of the term damping, a more scientific term energy dissipation can be used. A

damper dissipates the (undesired) energy which is introduced into the structural system. The

source of such undesired energy can be in earth quake, or, like in this case, in a combination

of rain and wind that excites a stay cable to vibrations that might be dangerous for the

structural system

What is the tolerable level of an

amplitude?

According to the FIB Bulletin 30/2005,

pp.21, the maximum amplitude of a

vibrating cable shall be kept below

1/1700 of its length. Should theamplitude Y1 of the (undamped) cable

exceed L/1700, a cable damper has to

be devised.

Fatigue of the cable

Next to the reduction of vibrations of big

amplitudes, another motivation for the

installation of dampers is the reduction

of small vibrations, which can result in

an early ageing of the material at the

anchor head caused by big curvatures.

Great care has to be taken that these

curvatures are not shifted to the locationof the damper.

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D i s p la c e m e n t [ m m ]

Force

[k

Energy dissipation means that a certain response force which is created by the damper

has to perform a certain displacement, and the product of force and displacement is theenergy that will be dissipated. The force could be a friction force (as we have it in the

friction dampers) or a shear force (which is the case in elastomeric dampers), or a viscous

response force (viscous dampers).

The energy dissipation capacity per cycle is expressed in the hysteresis of the damper. The

bigger the area that a hysteresis comprises, the better is its energy dissipation capacity.

F o r c e - D i s p l a c e m e n t - D i a g r a m 0 , 2 H z - 2 0 m m

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Hysteresis of a viscous damper (above)

andan elastomeric damper (below),

illustrating the superior energy

dissipation capacity of viscous dampers.


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Example of an externally arranged

viscous cable damper

(Eiland Bridge, Kampen, NL).

Why cant we use a conventional

damper ?

Such shock absorbers are certainly

cheaper than the tailor made cable

dampers. However, their design response

force will not match the requirement of

the response force of cable damperswhich can reach a magnitude of 40 kN

or more.

Due to the fact that the optimum

damping coefficient that is to be set is

different in each case, an individually

adjusted type with preset properties

would be required, when using vehicle

shock-dampers. In regard to delivery

date, costs, consulting, dimensions,

design of connections, installation,

removal and inspection possibilities this

is only difficult to accomplish. Long-term

behaviour of such vehicle shock-absorbers in regard to leakage, heat

transfer and effectiveness is also difficult

to judge (and to guarantee). Further,

they are no maintenance-free elements.

What kinds of cable dampers are

available ?

All cable dampers have in common that

they are attached to the stay cable close

to their end, at the level of the bridge

deck. At that location they will enact some

force onto the vibrating cable, thus

reducing the amplitude of the vibration.

Cable dampers can be arranged internally,

that is within the protection tube, or

externally. Internal cable dampers are

usually cheaper than the external ones,

and will not affect the aesthetics of the

bridge, and they may offer also more

safety against vandalism or other external

impacts. However, due to their relativecloseness to the stay cable their response

force capacity is limited, and so is their

damping capacity that such internal stay

cable dampers can introduce into the

system. This is why internal cable dampers

have usually a limited scope of

application, like in foot bridges, where the

length of the stay cables is limited, and

the damping requirement can be met with

such internal dampers.

Internal elastomeric Cable Damper in the test rig, showing the shear

deformation

External elastomeric cable damper

installed at Forchheim Bridge

The elastomeric cable damper

Such a damper works by shear

deformation of the high damping

elastomer. Elastomeric cable

dampers display a relatively weak

damping characteristic, which

mirrors the relatively low dampingcharacteristics of the elastomer.

Elastomeric cable dampers may be

useful for pedestrian bridges where

the stay cables are relatively short.

Their biggest advantage is their

low price, and in the same time

their weak technical performance is

their biggest draw back.

The friction damper

A friction damper dissipates energy

by means of friction forces that

perform a displacement which

corresponds to the amplitude of thestay cable at that location. Friction

dampers can employ a sufficiently

high damping ratio, however their

functionality is limited to a

relatively narrow range where there

is the maximum requirement for

damping. Below and above this

range, a friction damper will not be

very effective.


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Optimum Damping

In order to damp a certain vibration, that is a certain mode (most often the 1 stmode), we can imagine that there is a certain force acting on the cables with the

objective to reduce the amplitudes.

Is this force too weak, it does not create enough resistance to the vibrations,

and so the vibrations will not decay in the optimum way. In the other extreme,

should this force be too strong, at the location of the damper there will be no

movement of the cable. The damper clamps the cable, and no energy will be

dissipated. Since this point now turns to a node, the length of the cable that can

now freely vibrate is reduced by the length where the damper clamps, and the

cable in its reduced length can again be excited to vibrations.

In between there is a certain force that reduces the vibrations, i.e. the

amplitudes of the cable in an optimum way. This state is called the optimum

damping Each modal shape has its own state of optimum damping, and the

optimum response force of the cable damper may vary for each modal shape

that has to be damped.

Adaptive Cable Dampers ACDs)

As its name implies, an adaptive linearviscous damper can adapt its response

force to the modal shapes of the vibrating

cable, thus making sure that always

optimum damping is enacted. . The

damper thus adapts to the existing

situation on site, taking into account the

specific parameters that make the cables

vibrate, and tunes itself optimally to the

situation by way of a certain algorithm

Maurer Magneto-Rheological Dampers

(MRDs) can be considered to be a member

of the family of adaptive cable dampers.

MRDs behave under a constantcurrent in

a very good approximation like a friction

damper. This means that the electrical

current has to be regulated such that the

damper force behaves in a way that the

MR damper always works in its best

location (which is the optimum described

as Range 2 as explained in the section

about friction dampers) In this best

location the MR damper puts the

maximum possible damping into the

cable. This means that the MRD force

must be controlled proportional to thevibration amplitude at the location of the

MRD.

Hysteresis of an adaptive magneto-

rheological damper at different currents

Principle sketch depicting the arrangement of the coils around the piston,

for designing a semiactive magneto-rheological (MR) damper

In contrast to a conventional friction

damper: the controlled MRD prevents

clamping (which is 1stpriority), and by

way of an approximately optimum

control algorithm the controlled MRD

damps the cable in an optimum way.

In any case, the damping effect is

superior to the one of a conventional

friction damper. Because, when the

damper is off, the coupling between

cable and bridge deck is relatively

weak and thus vibrations of the deck

cannot excite vibrations in the cable,

which consequently reduces the cables

exposure to fatigue.

Adaptive Cable Damper (Eiland

Bridge, Kampen, NL)


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0 50 100 150 200 250 300

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signal of accelerometer

@ 24% cable length

after double integration

no MR damper, free cable

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r[mm]


@ 24% cable length


MR damper current I=0.00 A

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@ 24% cable length


MR damper current I=0.50 A

Decay of vibrations of an undamped cable,

i.e. no cable damper attachedMinimum decay of vibrations, when the

attached adaptive Cable damper is in

its base friction,i.e. no current. I = 0,

off-mode.

Optimum damping of an adaptive

cable

damper: fast decay of vibrations.

Damper is switched on, I = 0.5 A.

Special Features of Maurers

Adaptive Cable Damper

Maurer has designed the adaptive

cable damper (ACD) on the principle

of the magneto-rheological (MR)

fluid damper. The fluid displays the

characteristics that its shear strength

can be varied under the influence of

a magnetic field. Within the damper,

coils are arranged which by way of

electronic control can create a

magnetic field.

Decay Curves

The effect of an adaptive magneto-rheological cable damper can be

seen from the 3 figures above, which

were taken from measurements at

the Ijssel Bridge near Kampen, NL

that were conducted by EMPA in

Switzerland.

The figures depict the decay of the

amplitudes of a stay cable over the

time, after the stay cable was excited

by means of a rope in its natural

frequency. Left depicts the decay of

the undamped cable, that is without

a cable damper. The effect of an

inactive cable damper can be taken

from the central figure. Inactive

hereby refers to the state when the

magneto-rheological damper is

switched off, that is the current is

0. This state which is also called the

passive mode provides the minimum

damping effect of an MR damper.

The right figure above represents the

state of optimum damping,

Evaluating the performance of a Maurer

adaptive cable damper (ACD) in the test

rig of the EMPA located in Dubendorf,

Switzerland.

Power Supply of an adaptive

magneto-rheological damper

The current that is required to control

the magnetic field in the damper

takes a low energy supply. Should no

fixed power supply be available, solar

panels might suffice to provide the

necessary energy. In the said

example of the Ijssel Bridge near

Kampen, NL, the power consumption

is 21 mA when in passive mode, and

220 mA when in active mode. This

demonstrates that solar panels might

very well suffice.

The higher the electrical current, the

stronger the magnetic field, and thehigher the damper force (see Fig. 9). The

force of an MR damper has a lower and an

upper limit:

The lower limit is set by the

base friction at 0 Ampere

current and by the friction at

the sealing

The upper limit is set by the

maximum current

At a constant current, in good

approximation the damper force follows

the Coulomb Friction, this means the force

is independent from the velocity of

vibration. Due to the effect of base

friction, an MR damper already employs a

damping effect when the electric current

is switched off, or when there is a power

break down.


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Reference Projects

Sutong Bridge / China

MAURER ACD,

Response Force variable from 1 50 kN,

Stroke 65 mmSupply of a total of 200 cable dampers,

both of type ACD and passive linear

viscous dampers.Longest cable to be damped is 588 m.

Eilandbrug Kampen / Netherlands

Test installation of MAURER Adaptive Cable

Damper:

Response Force variable from 1 40 kN,

Stroke 60 mmLength of damped cable = 163 m.

Bridge Dubrovnik / Croatia

Response Force variable from 1 40 kN

Stroke 80 mm

Supply of 20 adaptive cable dampers (ACDs)

Maurer Shne Head OfficeFrankfurter Ring 193, D-80807 Mnchen

P.O. Box 44 01 45, D-80750 Mnchen

Phone: ++49/89/32394-0

Fax: ++49/89/32394-306

[email protected]

www.maurer-soehne.de

Maurer Shne Main Branch OfficeZum Holzplatz 2, D-44536 Lnen

P.O. Box 63 40, D-44520 Lnen

Phone: ++49/231/43401-0

Fax: ++49/231/43401-11

Maurer Shne Subsidiary PlantKamenzer Str. 4 6, D-02994 Bernsdorf

P.O. Box 55, D-02992 Bernsdorf

Phone: ++49/35723/237-0

Fax: ++49/35723/237-20

Prospekt MAURER Seildaempfer Englisch

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